1. Field of the Invention
The present invention relates to an aqueous gel ink-filled ball point pen, more specifically to a ball point pen filled with an ink having properties expressed by pseudo-plasticity, non-Newtonian viscosity or shear thinning viscosity.
2. Description of the Prior Art
A ball point pen is composed of a pen tip comprising a ball and a tip holder, an ink reservoir and a barrel. In writing with a ball point pen, the ink flowing out of the inside of the tip with rotation of the ball is transferred on or penetrated into a recording material such as paper, whereby letters and lines are written.
An aqueous ink-filled ball point pen has the advantage that it uses an ink having a low viscosity of several mPa.cndot.s and therefore can write at a low writing pressure and has a good writing feeling. However, it has the defect that natural outflow of the ink from the point of the tip, a so-called point seepage (hereinafter called a direct flow phenomenon) or a back-leaking phenomenon in which air flows in from the tip of the ball point pen to allow the ink to flow out from the ink reservoir is liable to take place. A method in which an ink absorber such as a bundle of fiber is used is employed to prevent these phenomena.
Further, there is the problem that if the ball point pen is left in a cap-off condition, the solvent is dried up because of the high vapor pressure of the solvent, and therefore the pen point is dried to prevent the ink from flowing out, so that the ball point pen becomes incapable of writing.
On the other hand, a conventionally known oil base ball point pen uses an ink having a high viscosity of several thousand mPa.cndot.s and therefore has the defect that the ball receives large resistance in rotating when the ink flow out of the pen tip to deteriorate a writing feeling. Further, there are the problems that the less amount of the ink flows out of the tip in writing to cause a blobbing phenomenon and bring about unevenness on drawn lines and that the line intensity is weak and therefore a high writing pressure is required.
In order to improve the oil base ball point pen, a ball point pen for an aqueous ink having a so-called medium viscosity which falls within a middle viscosity region (several mPa.cndot.s to several thousand mPa.cndot.s) ranging between the viscosities of an aqueous ink and an oil base ink has recently been developed. This is a ball point pen using a relatively low viscosity aqueous ink having the characteristic that the viscosity of the ink is lowered by the rotation of the ball point to allow the ink to flow out smoothly, a so-called shear thinning viscosity.
This ink is called a gel ink, and the property thereof is shown by expression such as pseudo-plasticity, non-Newtonian viscosity and the like in a certain case.
However, if this ink has too strong property as gel, the flowability of the ink is inferior, and the outflow of the ink from the pen point is deteriorated in writing, so that starving and inferior writing are caused. Too much outflow brings about feathering and blobbing phenomena in some cases. On the other hand, if gelation is weak and the viscosity is low, the phenomenon that the ink is liable to bring about back flow or direct flow is observed. Accordingly, writing feeling, back flow and direct flow phenomena and pen structure exert influence on each other, so that an ideal aqueous gel ink-filled ball point pen has not yet been obtained.